1. Field of the Invention
The present invention relates to a signal processing apparatus for a photo-semiconductor position sensing device, and more particularly to a distance measuring device for a camera which uses a semiconductor position sensing device (PSD).
2. Description of the Prior Art
A distance measuring device for measuring a distance by a principle of triangulation based on a predetermined distance (base line length) and an angle between a light emitting element and a photosensing element by arranging the light emitting element and the photosensing element in spaced relation from each other by the base line length and scanning one of the light emitting element and the photosensing element so that the light emitted by the light emitting element and reflected by an object is sensed by the photosensing element, has been well known. However, this method needs a mechanism for scanning the light emitting element on the photosensing element, and when the distance measuring device is used in the camera, it is necessary to synchronize the drive of a distance ring of an imaging lens to the scan. Accordingly, the distance information cannot be obtained prior to shutter release operation. As a result, prefocus photographing in which a distance to an object in a view field of a finder is measured prior to the shutter release operation is hard to attain.
In this respect, a distance measuring device which requires no scan mechanism and allows prefocusing has been known. It uses an array of a plurality of light emitting elements or photosensing elements instead of scanning the element and measures the distance by the principle of triangulation. However, the arrangement of the plurality of light emitting elements or photosensing elements not only increases the cost but also complicates the circuit for driving the light emitting elements and/or complicates the detector for detecting signals from the plurality of photosensing elements. Accordingly, this method is hard to apply to a compact camera.
A camera which incorporates a distance measuring device as disclosed in Japanese Patent Application Laid-Open No. 95210/1983 has also been known. In this Patent Application, a light emitting device for projecting a light to an object and a semiconductor position sensing device (PSD) for producing a first and second current outputs having a mutual current ratio determined by an incident position of the light emitted by the light emitting device and reflected by the object are provided so that the distance is measured based on the first and second current outputs.
This distance measuring device is advantageous in that the PSD is relatively inexpensive and a processing circuit therefor is simpler than that for the plurality of photosensors. However, it has the following disadvantage.
In the prior art method, the output current of the PSD is affected by the intensity of the spot light impinged on the PSD such that the output current is high when the light reflected by the object is strong and the output current is low when the length reflected by the object is weak. Accordingly, it is necessary to normalize the output current of the PSD by dividing it by a sum of the first and second current outputs of the PSD. Thus, the signal processing circuit is complex and a wide dynamic range of the signal processing circuit is required because the output current of the PSD is significantly changed with the intensity of the incident spot light even though the output current of the PSD is normalized by the sum of the first and second current outputs of the PSD. Accordingly, if the signal processing circuit is constructed by discrete components, the selection range for an operational amplifier is narrowed, and if it is constructed by a monolithic chip, the circuit is further complicated because a signal compression circuit is required to widen the dynamic range.